The present invention relates to a circuit for generating an output voltage which is proportional to temperature with a required gradient.
Such circuits exist which rely on the principle that the difference in the base emitter voltage of two bipolar transistors with differing areas, if appropriately connected, can result in a current which has a positive temperature coefficient, that is a current which varies linearly with temperature such that as the temperature increases the current increases. This current, referred to herein as Iptat, can be used to generate a voltage proportional to absolute temperature, Vptat, when supplied across a resistor.
Although this principle is sound, a number of difficulties exist in converting this principle to practical application.
One such practical difficulty is the need to maintain a stable internal line voltage in the face of significant variations in a supply voltage. This should be done without unnecessarily increasing the number of components in the circuit over and above those which are required to generate the voltage proportional to temperature.
The present invention provides a circuit for generating an output voltage proportional to temperature with a required gradient, the circuit comprising: first and second bipolar transistors with different emitter areas having their emitters connected together and their bases connected across a bridge resistive element, wherein the collectors of the transistors are connected to an internal supply line via respective matched resistive elements such that the voltage across the bridge resistive element is proportional to temperature; a differential amplifier having its inputs connected respectively to said collectors and its output connected to a control terminal of a first control element having a controllable path connected between a first power supply rail and a control node; a second control element having a controllable path connected between the control node and a second power supply rail; and a third control element having a control terminal connected to the control node and a controllable path connected between the second power supply rail and an internal supply line, whereby the differential amplifier and the first, second and third control elements cooperate to maintain a stable voltage on the internal supply line despite variations between the first and second power supply rails.
In the described embodiment the stable voltage on the internal supply line is used to power components of a second stage which allows fine adjustment of the predetermined gradient of the voltage proportional to temperature.
In the described embodiment, the voltage on the internal supply line is set from the voltage proportional to absolute temperature using that voltage in conjunction with two bipolar transistors connected in series via a resistor to an output node at which a voltage proportional to absolute temperature with a predetermined gradient is generated.
Thus, the embodiments of the invention described in the following focus on line regulation of a circuit such that if the supply voltage to a chip increases, the output of the temperature sensor does not change (or only very minutely). This is done by having a constant internal supply line for the major circuitry which is quite stable with temperature. If this does not change, then the assumption can be made that the local supply (Vddint) is constant.
In the following described embodiments, three components in particular are discussed:
(i) The value on the internal supply line (Vddint) is set by the voltage across the bridge resistive element and two bipolar transistors connected in series, using the current proportional to absolute temperature which is generated in the circuit.
(ii) The drop of voltage between the first and second power supply rail and the internal supply line (Vddint) appears across the collector/emitter of the third control element. The bias for that control element is provided by the first and second control elements.
(iii) The third control element also can provide the current supply for the internal supply line. Any disturbance of current or voltage on the internal supply line loops back through the resistive bridge element, xcex94Vbe generator, differential amplifier to the first and second biasing control elements and to the third control element.